Visible light emission due to the hot electron dynamics is widely observed in high power pseudomorphic high electron mobility transistors ( PHEMT ). Most likely, it is associated with the impact ionization of the channel region and device breakdown. As a result, it is very crucial to identify where the light emission comes from and the spectrum of the light to further improve the device power performance. In the experiments, we used LAMMP facility to examine the light source and light spectrum on the PHEMT devices biased at the breakdown region of gate-source, gate-drain, drain-source and hot carrier operation. From the light emission image and spectrum, it is possible to examine the light source and lighting mechanism with this high resolution equipment. Collaboration with LAMMP scientists may facilitate development of PHEMT-based biosensors.